The first child to receive a new experimental gene therapy for a rare neurological disorder died a few days after dosing, the company sponsoring that trial said Wednesday. The company, Capsida Biotherapeutics, is one of a group of companies and labs engineering new viruses potentially capable of safely ferrying genes deep into the brain. Its first effort was designed to treat STXBP1 encephalopathy, a condition that can cause seizures, developmental delays, and other symptoms. The STXBP1 Foundation said in a note to community members that the company and investigators were still working to understand what happened.